Circumventing the diffraction limit of light using scattering-type scanning optical microscopy (S-SNOM) has proven to be a powerful technique for probing the local nanoscale optical properties of solids. Its recent applications as a nano-imaging tool have employed mid-infrared frequencies while circumventing the diffraction limit by nearly three orders of magnitude. By using broadband illumination with asymmetric Fourier transform infrared (FTIR) spectroscopy, observation of the local near-field spectra with nanometer scale spatial resolution has been realized. S-SNOM is based on operating an atomic force microscope (AFM) in tapping mode in which light is focused on to the AFM's metallic tip in close proximity to a sample being examined. The tip-sample interaction is encoded in the scattered signal which is then measured with a detector. Since the ratio of the scattered signal to incoming light at the tip is so small, it is necessary to use high intensity light sources.
Broadband nano-spectroscopy in the far and mid-infrared spectral range is challenging because of the limitations of existing high-intensity light sources. The use of tunable, monochromatic lasers allows for high signal strength, but is limited by the available wavelengths and by the amount of time that it takes to obtain a high-resolution broadband spectrum. Quantum cascade lasers (QCLs) have been implemented with S-SNOM and have the ability to quickly scan through wavelengths, but have a narrow spectral range. Difference frequency generation provides a stable high-intensity beam in the mid-infrared, but needs to be tuned to different wavelength ranges to get the full spectrum and currently has a low frequency cutoff of approximately 550 cm−1. Thermal blackbody light sources like the globar provide a large spectral bandwidth, but only at low intensities. Hence, significant integration time is required to obtain data with a globar and there is no usable intensity below approximately 750 cm−1 for broadband S-SNOM. Synchrotron light sources provide spatially coherent intense broadband light that is currently the highest intensity and widest bandwidth infrared source for nano-spectroscopy. However, synchrotron systems are large and expensive systems. Furthermore, there are only a handful of synchrotrons in the world that have far-infrared and mid-infrared beamlines. Accordingly, access to far-infrared to mid-infrared beam lines is competitive such that they not readily available for more time-consuming experiments.
The most common type of commercial plasma light sources are the xenon-filled high-pressure plasma lamps which are useful as the broadband source for the near-infrared, visible, and ultraviolet spectral ranges, i.e., all having frequencies higher than 2,500 cm−1. However, these lamps do not provide intensity in the mid and far-infrared since the plasma is encased in a quartz bulb that is opaque to these wavelengths.
Infrared spectroscopy has been commonly used to probe infrared-active phonons and charge dynamics in materials. However, many materials have been shown to exhibit phase coexistence at length scales much smaller than the diffraction limit of infrared light. Infrared nano-spectroscopy techniques are necessary to properly understand the charge and lattice dynamics of these nano-domains that exist in a number of materials. To have the ability to probe nanoscale domains with broadband infrared spectroscopy in the far-infrared and mid-infrared spectral ranges would allow these types of experiments to be performed on a number of materials to discover and explore nanoscale phenomena that may also have significant potential for applications. Researchers will be able to probe the crystallinity of thin films over a broad spectral range, allowing the testing of the effectiveness of different growth methods. Moreover, the technique can be employed for nanoscale identification of materials, and quality control and characterization of nano-devices.
To summarize, S-SNOM allows spectroscopic investigation of materials at length scales much smaller than the diffraction limit of light. Accordingly, S-SNOM has enormous potential as a spectroscopy tool in the infrared spectral range where it can probe phonon resonances and carrier dynamics at the nanometer length scales. However, S-SNOM processes are limited by the lack of practical and affordable table-top light sources emitting intense broadband infrared radiation in the 100 cm−1 to 2,500 cm−1 spectral range indicative of the far to mid-infrared spectral range.